Die

ABSTRACT

This invention is addressed to a new and improved die for use in extrusion and drawing operations which includes an annular casing, a die nib which is shrunk fit into a central opening in the casing and means exerting a compressive force on the die nib in the axial direction whereby the die nib is subject to both radial and axial compressive stresses.

United States Patent Verduzco et al.

DIE

Inventors: Miguel A. Verduzco, Chicago;

James W. Dally, Park Forest, both of III.

LaSalle Steel Company, Hammond, Ind.

Filed: Aug. 10, 1970 Appl. No.: 62,255

Assignee:

US. Cl 72/467; 76/107 A Int. Cl. B21C 3/00 Field of Search 72/467;76/107 A; 29/447 References Cited UNITED STATES PATENTS 5/1944 Fenner72/467 Nov. 11, 1975 2,647.847 8/1953 Black 29/447 3,014,583 12/l96lHuffman 72/467 X 3,360,846 H1968 Schellstede 29/447 Primary E.\'aminerC.W. Lanham Assistant ExaminerRobert M. Rogers [57] ABSTRACT Thisinvention is addressed to a new and improved die for use in extrusionand drawing operations which includes an annular casing, a die nib whichis shrunk fit into a central opening in the casing and means exerting acompressive force on the die nib in the axial direction whereby the dienib is subject to both radial and axial compressive stresses.

1 Claim, 10 Drawing Figures US. Patent Nov. 11, 1975 Sheet 10123,918,288

N VE N T025 Miguel gezcziuzco mes a US. Patent Nov. 11, 1975 Sheet20f23,918,288

CL I

DIE

This invention relates to drawing and extrusion dies, and moreparticularly to drawing and extrusion dies which are radially andaxially prestressed.

In cold drawing operations, it has been common practice for many yearsto make use of compound dies with the work section of the die, oftenreferred to in the art as the nib, formed of a hard brittle materialsuch as tungsten carbide, shrunk into a metal casing. Such casings areusually formed of an annular piece of metal with the central openingbeing dimensioned slightly less than the outer dimension of the nib.Thus, the casing is heated to a temperature sufficient to causeexpansion of the casing whereby the nib can be inserted into the centralopening. As the metal casing cools, it contracts, thereby subjecting thenib in the central opening thereof to relatively high compressivestresses in a radial direction.

The compressive stresses developed in the radial direction of the nibduring the shrinkage described above serve to prevent failure of the dieby crack propagation along a plane perpendicular to the circumferentialdirection.

However, experience in cold and warm drawing and extrusion operationshas indicated that radially prestressed dies may also fail by fracturealong a plane perpendicular to the axial direction. Such failures canbecome a serious problem in drawing and extrusion, particularly duringwarm drawing and/or extrusion operations in which the effect oftemperature combined with excessive drawing or extrusion stressesresults in a high incidence of die failure.

Photoelastic stress analyses of drawing dies have shown that largetensile stresses develop at the entrance of the die, and at transitionsin geometry along the die contour. Without limiting the presentinvention as to theory, it is believed that these stresses are relatedto failure and accelerated wear of dies. In addition, it has been foundthat thermal shocks can be developed on the surface of the die in thecourse of warm drawing and extrusion operations. Thus, is is believedthat the combined effect of thermal shocks and drawing or extrusionstresses is at least in part responsible for crack propagation, andhence die failure.

In accordance with the present invention, it has been found that diefailure and acceleratered die wear can be prevented or substantiallyminimized by means of radially and axially prestressed dies.

Radially and axially prestressed dies are known to the prior art asrepresented by US. Pat. No. 3,248,972.

However, in the die disclosed in this patent, the axial stresses whichcan be developed are somewhat limited by the geometry of the die andcasing assembly, such that only limited advantages can be derived,particularly during warm drawing or extrusion operations. In addition,the prestressed die disclosed by this patent has a relatively complexgeometry which contributes undesirably to manufacturing costs and toease of assembly.

It is accordingly an object of the present invention to provide a newand improved prestressed die which overcomes the foregoingdisadvantages.

It is a more specific object of the invention to provide a radially andaxially prestressed die which prevents or substantially minimizes diefailure and/or accelerated die wear, even during warm extrusion anddrawing operations.

It is yet another object of the invention to provide a radially andaxially pre-stressed die in which use is made of mechanically and/orthermally induced stress.

These and other objects and advantages of the invention will appearhereinafter, and, for purposes of illustration, but not of limitation,embodiments of the invention are shown in the accompanying drawings inwhich:

FIG. 1 is a schematic sectional view on one side of the axis of symmetryof a die embodying the features of the invention;

FIG. 2 is a top view of the die illustrated in FIG. 1;

FIG. 3 is an exploded schematic sectional view of another embodiment ofthe invention taken along one side of the axis of symmetry;

FIG. 4 is a top view of the die illustrated in FIG. 3;

FIG. 5 is an alternative embodiment to that shown in FIGS. 3 and 4;

FIG. 6 is a schematic sectional view of yet another embodiment of theinvention on one side of the axis of symmetry;

FIG. 7 is a schematic sectional view of another embodiment of theinvention on both sides of the axis of symmetry;

FIG. 8 is a partial sectional view on both sides of the axis of symmetryof the stressing ring element of the embodiment of FIG. 6;

FIG. 9 is a schematic sectional view of another embodiment of theinvention in which use is made of hydraulic pressure to induce axialstress in the die; and,

FIG. 10 is a schematic sectional view of another embodiment of theinvention similar to that shown in FIG. 8 in which use is made ofthermal means to induce axial stress in the die.

The concepts of the invention reside in a die comprising an annularcasing and a die nib shrunk fit into the central opening of the casingwhich gives rise to the desired radial prestressing of the nib. Axialprestress of the nib can be induced through mechanical and/or thermalmeans in a simple and efficient manner whereby die failure and/oraccelerated die wear can be prevented or substantially minimized.

Referring now to the drawings, there is shown in FIG. 1 one embodimentof the invention in which use is made of mechanically and thermallyinduced stress in the axial direction. The die schematically illustratedin FIG. 1 is shown only on one side of the axis of symmetry or centerline CL for the sake of simplicity, with the understanding that thecross section of the die on the other side of the axis of symmetry isidentical.

The die comprises an annular casing including an annular body 10defining a substantially cylindrical central opening or bore 12 adaptedto receive in a shrink fitting relationship a nib 14 having asubstantially cylindrical outer surface 16 having a diameter slightlylarger than the diameter of bore 12 cylindrical abutting the surface ofcentral opening 12. Nib 14 may have a variety of geometricconfigurations for effecting drawing or extrusion operations. As shownin FIG. I, the nib 14 has an approach surface 18, a blend surface 20 anda bearing surface 22. For a more detailed description of the internalgeometry of the die nib, reference can be made to US. Pat. No.3,157,274. However, it will be understood by those skilled in the artthat a wide variety of other die nibs suitable for the drawing andextrusion of metals can similarly be employed in the practice of thepresent invention.

Nib 14 has an axial dimension which is somewhat greater than thethickness of body whereby the upper and lower portions 24 and 26,respectively extend axially beyond the body 10. The nib 14 and easingbody 10 are sandwiched between a pair of annular plates 28 and 30, andsecured by means of a plurality of bolts 32 adapted to be thermallyprestressed spaced radially about plates 28 and 30 and extending in anaxial direction through a bore 34 in plate 28 and bore 36 in body 10.Bolts 32 may be secured by, for example, means of a threaded bore 38 inlower plate 30. The spacing of bolts 32 about the plate 28 can best beillustrated by reference to FIG. 2 of the drawings. Thus, it will beapparent that the bolts 32, as they are tightened, operate to exert acompressive force on the portions 24 and 26 of nib 14 in abuttingrelation with plates 28 and 30, respectively, to thereby axiallyprestress the nib 14.

In order to insure that the compressive forces exerted by plates 28 and30 be uniformly distributed over the surfaces of nib 14, it is preferredthat plates 28 and 30 be in as close to a parallel relation as possiblewhen the nib 14 is in a state of compression. For this purpose, it isgenerally preferred to provide body 10 with a pair of annular flangeelements 40 and 42 about the periphery of body 10 to abut the internalsurfaces of plates 28 and 30, respectively.

In assembling the die illustrated in FIG. 1, body 10 is heated to anelevated temperature sufficient to cause thermal expansion thereofwhereby nib 14 can be fitted into the central opening 12 with portions24 and 26 extending in an axial direction beyond the upper and lowersurfaces of body 10, respectively, as described and shown. Thereafter,the body is allowed to cool, and plates 28 and 30 are positioned,preferably after surfaces 24, 26, 40 and 42 have been ground to removeany oxides formed ,on these surfaces during the heating operation. Bolts32 are then heated, threaded in position as shown in FIGS. 1 and 2 andtightened to a predetermined torque.

As bolts 32 cool, they contract thereby subjecting nib 14 to furthercompressive forces in addition to those developed by the tightening ofbolts 32 at an elevated temperature. Thus, the die nib 14 is subjectedto radial compression as a result of shrinkage from the first heatingstep, and to axial compression as a result of the tightening and coolingof bolts 32 in the second step. It has been found that the heating ofthe body 10 and the bolts 32 should be carried out in separateoperations to insure the development of maximum compressive forces.

As will be appreciated by those skilled in the art, the axial stressesto which the nib is subjected may be conveniently controlled byregulating the torque applied to bolts 32 and the temperature to whichthe bolts are heated. For most applications, preheating temperatures of1,000 to 2,000 F. for the bolts are sufficient.

Another embodiment of the invention is illustrated in FIGS. 3 and 4 ofthe drawings, in which use is made of thermally induced stress. Again,the drawings illustrate the die on one side of the axis os symmetry orthe center line. As shown in FIGS. 3 and 4, the die includes a pair ofcomplementary semi-annular casing elements 50 and 50 which can besecured together by means of, for example, a plurality of bolts 52extending through casing elements 50 and 50 by way of bores 54 in casingelements 50 and 50.

Casing elements 50 and 50 together define a central opening bounded by acylindrical surface 56 on the interior of the casing elements having adiameter which is slightly less than the diameter of a nib 58 having anouter corresponding cylindrical surface 60. Casing elements 50 and 50are also provided with a pair of axially spaced flange members 62 and64. Flange members 62 and 64 are adjacent to cylindrical surface 56 andtherewith define a central recess 66 adapted to receive the nib 58 in ashrunk fitting relationship in both the axial and radial directions.

Shrink-fitting in an axial direction is insured by the fact that flangemembers are spaced such that the distance between their internalsurfaces is slightly less than the axial dimension of nib 58.

Thus, in assembling the die of this embodiment, the casing elements areheated to an elevated temperature sufficient to cause expansion of thecasing elements to enable the nib 58 to be inserted in the centralopening defined by cylindrical surface 56 and flange elements 62 and 64.After the nib is inserted, the casing elements are positioned to enclosethe nib and allowed to cool, thereby developing an axial compressivestress on the die nib 58. Thereafter, the bolts 52 are heated andthreaded into the cold casing and allowed to cool. The bolts 52 thuscontract on cooling to introduce a radial stress to nib 58. As will beappreciated, the development of radial stresses will also serve toslightly increase the axial stresses already induced as described above.

Using this concept of the invention axial stresses as high as 150,000psi can be developed, depending on the strength of the flange elements.Best results are usually achieved when the axial dimension of the nibexceeds the distance between the internal surfaces by an amount from0.005 to 0.050 inches. The temperature to which the casing elements areheated will, of course, depend upon the relative dimensions of the niband casings. For the dimensions described above, it is generallysufficient to heat the casing elements to a temperature within the rangeof l,O0O and 2,000 F.

An alternative embodiment of the die assembly shown in FIGS. 3 and 4 ofthe drawing is described in FIG. 5 of the drawing, and includes a dienib 58' and an inner casing 51 and 51' having a similar configuration tocasing 50 in FIGS. 3 and 4. Only casing element 51 is illustrated inFIG. 5 for convenience. Inner casings 51 and 51 are provided with asubstantially cyclindrical outer surface 65. Instead of using bolts inthis embodiment to maintain the casing elements 51 and 51 together, useis made of an annular ring 63 having an inside diameter which isslightly less (by, for example, tolerances of from 0.005 to 0.100inches) than the outside diameter of casing elements 51 and 51.

In this embodiment, the casing elements 51 and 51' are heated andpositioned to enclose nib 58. After the casing elements 51 and 51' havecooled, ring 63 is heated to a temperature sufficient to enable theassembly of casing elements 51 and 51' enclosing nib 58' to be insertedinto the central opening thereof. Thus, in this embodiment radial stressis induced by means of the annular ring 63 and axial stress by thecasing elements as in the embodiment of FIGS. 3 and 4.

Another embodiment of the invention, which makes use of mechanicallyinduced axial stress is illustrated in FIG. 6 of the drawing ln thisembodiment 'thesdie is .2.

formed of an annular casing having an annular-flange element 72extending radially inwardly. The innei s ur face of casing 70 adjacentto flange element 72 is formed of a cylindrical surface 74. Thus,annular flange 72 and cylindrical surface 74 form" arecess 76 which isadapted to receive a die nib 78 in a shr'u'rik'fit relation.

expansion to enable nib 108 to be inserted in recess thereby develop thedesired radial stress on nib 108.

As is the case of the preceding embodiments, the outer diameter of nib78 is slightly greater than the diameter of surface 74 whereby the nib78 is fitted into recess 76 while casing 70 is at an elevatedtemperature.

The end of casing 70 opposite flange 72is provided with a secondgenerally cylindrical recess 80 having a greater diameter than therecess defined by surface 74 which is adapted to receive an annularretaining ring threadably engaged with casing 70 by means of threads 84.As shown in FIG. 6, retaining ring is superposed over the upper surface86 of nib 78. 'i v Retaining ring 82 is provided "with a plurality ofstressing bolts 88 spaced about the annulus of the ring and threadablyengaged in a bore 90 extending through retaining ring 82 and terminatingin a flat plate aligned with the upper surface portion 86ofnib 78 and inabutting relation therewith.

Thus, in assembling the die of this embodiment, the casing 70 is firstheated to a temperature sufficient to enable nib 78 to be inserted inrecess the nib inserted and assembly allowed to cool, thereby givingrise to the radial stress on nib 78 described above.

Thereafter, retaining ring carrying stress bolts in a raised position istightened into recess 80 whereby the plate portions 92 of bolts 88 arealigned with the surface 86 of nib 78. A stress can be applied to thenib by tightening bolts 88 whereby the plate portion 92 thereof exerts acompressive force on the nib in an axial direction. As will beappreciated by those skilled in the art, the stress applied to the nibcan be varied by varying the torque applied to bolts 88.

Another die in which use is made of mechanically induced stress is shownin FIGS. 7 and 8 of the drawing. In this embodiment, the die includes anannular casing 100 having a radially inwardly extending annular flangeelement near one end thereof. The inner surface of casing 100 adjacentto flange 102 is formed of a cylindrical surface 104. Thus, annularflange 102 and cylindrical surface 104 define a recess 106 adapted toreceive a die nib 108 in a shrink fit relation. As is the case in thepreceding embodiments, the outer diameter of nib 108 is slightly greaterthan the diameter of the recess defined by surface 104 whereby nib 108is fitted into recess 106 while casing 100 is at an elevatedtemperature. It will also be observed in FIG. 7 that the axial dimensionof nib 108 is slightly greater than the axial dimension of surface 104whereby the upper surface 1 of nib 108 positioned in recess 106 extendsaxially beyond cylindrical surface 104.

Adjacent to the top of cylindrical surface 104, casing 100 defines anannular groove 112, the lower surface 114 of which lies in a planecoincident with the radial.

plane defining the upper limit of surface 104. Groove 112 is adapted toreceive a stressing ring which is an annular ring 116, having the crosssection shown in FIG. 8, and a camber prior to insertion into groove 112defined by housing 100 in an axial direction extending away from the nib108.

In assembling the die of this embodiment, casing 100 is heated to anelevated temperature to cause sufficient Then the stressing ring 116 ispressed into groove 112 with the camberextending away from surface ofnib 108, with the help of the tool 117 shown in FIG. 7. Because thedeformation of the ring is large, pressing of the ring mustbe done whileit is heated to a temperature between l000 2100 F. Hot pressing permitsdeforming of the ring until it is placed in position. The position ofring 116 in groove 112 is shown by broken lines in FIG. 7.

Compressive loads or stress induced by means of a stressing ring inaccordance with this embodiment can behigh although the load may besomewhat localized. Thus, in this embodiment of the invention, stressesas high as 100,000 psi can be achieved.

An axially stressed die in which the axial stress can be easily variedis schematically illustrated in FIG. 9. In

5 this embodiment, the casing 120 has the same general configuration asthe embodiment shown in FIG. 6, including an annular flange element andan axially intermediate cylindrical surface 124 and a threaded surface126 opposite flange 122. Surface 124 and flange 122 define a recess 128adapted to receive a die nib 130 in a shrink fit relationship. Threadedsurface 126 defines arecess 132 which is adapted to receive a retainingring or plug 134 in threaded engagement.

Plug 134 defines a recess 136 which is axially aligned with an uppersurface 138 of nib 130 positioned in recess 128. A portion of recess 136is occupied by a hydraulic ram 140 having an inverted T-shaped crosssection with the base 142 in surface contact with surface 138 of nib130. If desired, ram 140 can be provided with one or more sealing means144, such as piston rings or the like, to insurea fluid tight sealbetween ram 140 and the side of recess 136. Recess 136 is also providedwith means 146 to introduce a hydraulic fluid to recess 136.

In assembling the die of this embodiment, casing 120 is first heated toenable nib 130 to be inserted into recess 128 and is then allowed tocool to thereby develop the radial stresses in the manner described.Thereafter, plug 134 is screwed into position with base 142 of ram 140in surface contact with surface 138 of nib 130. A hydraulic fluid isthen introduced to recess 136 to thereby cause ram 140 to exert acompressive force in an axial direction on nib 130 and create thedesired stress.

This embodiment of the invention is particularly advantageous in thatthe axial compressive forces exerted upon the nib. can be easily variedduring extrusion or drawing operations by varying the pressure of thehydraulic fluid. In this way, the optimum axial stress for eachapplication can be determined and maintained.

As will be appreciated by those skilled in the art, recess 136 in plug134 and the hydraulic ram 140 positioned therein may be continuousannular elements extending about mid-radius throughout plug 134 tothereby insure that uniform compressive axial forces will be exerted onthe surface 138 of nib 130. Alternatively, plug 134 can be formed with aplurality of recesses or pressure chambers 136, each with a separatehydraulic ram, uniformly spaced about surface 138 of nib 130.

A variation of the embodiment shown in FIG. 9 is schematicallyillustrated in FIG. 10 of the drawing in which use is made of a thermalram instead of a hydraulic ram. As shown in FIG. 10, plug 134 isprovided with a continuous or discontinuous bore or recess 148 which isadapted to receive a thermal ram 150 formed of a metal having a highcoefficient of thermal expansion having an inverted T-shaped crosssection with the base 142' in surface contact with surface 138' of nib130. Thermal ram 150 is provided with heating means formed in the ram,such as insulated heating coils 152 which are connected to a source ofelectric current (not shown) by means of leads 154.

Thus, once the die is assembled, a current is fed to the heatingelements to thereby heat the thermal ram, which in turn causes thermalexpansion of the ram. As will be apparent, expansion occurs in the axialdirection whereby base 142 of the ram exerts a compressive force onsurface 138' of nib 130 to create the desired axial stress.

In this embodiment of the invention, the amount of stress developed onthe nib can be conveniently varied to determine the optimum stress bycontrolling the amount of current supplied to the heating elements.

It will be apparent from the foregoing that we have provided new andimproved drawing and extrusion dies which can be simply and efficientlypre-stressed prior to use in both the radial and axial directions. Inmany of the embodiments described the casing and/or parts thereof inwhich the die nib is mounted for use are completely reusable when thenib has been worn, which represents a significant economic savingsbecause the casings are generally machined to close tolerances and aretherefore relatively expensive to manufacture. It will be apparent thatthe concepts of the invention are applicable not only to drawing andextrusion dies as described above, but also to other types of dies.

It will be understood that various changes and modifications can be madein the details of construction, procedure and use without departing fromthe spirit of the invention, especially as defined in the followingclaims.

We claim:

1. A die comprising a casing having axially spaced flanges extendinginwardly to define a central opening in the casing, a die nib having anaxial dimension greater than the spacing between the flanges of saidcasing and a diameter greater than the diameter of said central openingpositioned in said central opening whereby the die nib is adapted to bereceived in the central opening when the casing is at an elevatedtemperature whereby the nib in the central opening is subjected to axialand radial stresses on cooling of the casing, and a ring about thecasing having a central recess corresponding to the outer surface of thecasing, with the ring being adapted to receive the casing when the ringis at an elevated temperature whereby the ring ex erts a radial stresson the die nib on cooling.

1. A die comprising a casing having axially spaced flanges extendinginwardly to define a central opening in the casing, a die nib having anaxial dimension greater than the spacing between the flanGes of saidcasing and a diameter greater than the diameter of said central openingpositioned in said central opening whereby the die nib is adapted to bereceived in the central opening when the casing is at an elevatedtemperature whereby the nib in the central opening is subjected to axialand radial stresses on cooling of the casing, and a ring about thecasing having a central recess corresponding to the outer surface of thecasing, with the ring being adapted to receive the casing when the ringis at an elevated temperature whereby the ring exerts a radial stress onthe die nib on cooling.